a) Field of the Invention
The present invention relates to a compact zoom lens system consisting of three lens groups.
b) Description of the Prior Art
In the recent years where lens shutter cameras are designed for more sophisticated functions, it is strongly demanded to equip the cameras with zoom lens systems. Out of the zoom lens systems so far proposed to cope with this demand, there is known the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 184916/58. This lens system consists of a first lens group having positive refractive power, a second lens group having positive refractive power and a third lens group having nagative refractive power, and is so adapted as to vary focal length from the wide position to the tele position by displacing the lens groups independently toward the object side. Judging from the lens composition, this zoom lens system can be regarded as a modification of the zoom lens system consisting of two lens groups which was disclosed by Japanese Unexamined Published Patent Application No. 201213/57, and the second lens group is displaced for a very short distance for varying focal length of the former zoom lens system. Further, the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 78522/62 consists of three lens groups having the above-described refractive power distribution but, judging from the compositions of the respective lens groups and description in the patent specification thereof, it can be regarded as a zoom lens system designed by integrating a second lens group with a third lens group in a zoom lens system consisting of four lens groups.
Japanese Unexamined Published Patent Application No. 17422/63 disclosed a conventional example of zoom lens system wherein refractive powers are similarly distributed, and a first lens group and a third lens group are displaced for variation of focal length. However, this zoom lens system is of an optically compensating type, designed for a low zooming ratio of 1.5 and has a total length too long to be a compact zoom lens system.
In the conventional examples described above as zoom lens systems designed for a vari-focal ratios on the order of 1.7 and consisting of three lens groups, driving mechanisms for zooming are inevitably complicated for displacing the three lens groups independently. Especially when an attempt is made to enhance the vari-focal ratio, it is more difficult to design a compact zoom lens system having a composition compatible with a simple driving mechanism.
In addition, the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 184916/58 can hardly be built in a lens shutter camera since the zoom lens system has a long total length and an extremely short back focal length which makes diameter of the third lens group as long as the diagonal line of the film.